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Chapter 16

Chapter 16. Evolution of Populations. 16-1 Genes and Variation. Darwin and Mendel Genes control heritable traits Changes in genes = variation Natural selection works with this variation. Types of Genetic Variation. Different alleles Dominant, recessive, multiple alleles, etc.

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Chapter 16

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  1. Chapter 16 Evolution of Populations

  2. 16-1 Genes and Variation • Darwin and Mendel • Genes control heritable traits • Changes in genes = variation • Natural selection works with this variation

  3. Types of Genetic Variation • Different alleles • Dominant, recessive, multiple alleles, etc. • Heterozygous genotypes • Can pass on dominant OR recessive

  4. Gene Pool • Combined genetic info. of all members of a population • Relative frequency: # of times an allele appears in a gene pool compared to other alleles • Has nothing to do with dominant/recessive • Evolution = change in relative frequency of alleles in a population

  5. Sources of Genetic Variation • What is the biggest source? • 1. mutations! • 2. gene shuffling- ½ from mom and ½ from dad • Random gamete formation

  6. Single-gene and Polygenic Traits • Single-gene trait: controlled by one gene with 2 alleles • Ex: widow’s peak (2 possible phenotypes) • Polygenic trait: controlled by 2 or more genes • Ex: skin color (many possible phenotypes)

  7. 16-2 Evolution as Genetic Change • Evolutionary fitness- organisms success in passing genes on to offspring • Evolutionary adaptations increase ability to pass on a trait • Survive = pass genes on • Fail = do NOT pass genes on • Evolution acts on organisms NOT single genes! • Populations evolve NOT organisms

  8. Natural Selection and Phenotypes • Natural selection affects the distribution of phenotypes (bell curve) • 3 Types: • 1. directional selection: when individuals at one end of the curve have increased fitness

  9. 2. stabilizing selection: when individuals at the center of the curve have increased fitness

  10. 3. disruptive selection: when individuals at both ends of the curve have increased fitness • *may eventually split into 2 different species

  11. Evolution WITHOUT Natural Selection? • Genetic drift: random change in allele frequency that occurs in a small population • By chance certain alleles are passed on over others • Founder effect: change in allele frequency due to the migration of a small subgroup of a population

  12. Will evolution ever NOT happen? • Hardy-Weinberg Principle: allele frequency in a population will remain constant as long as the following factors take place: • 1. random mating • 2. large population • 3. no movement in/out of a population • 4. no mutations • 5. no natural selection • So, does this ever happen? • No, not really

  13. 16-3 The Process of Speciation • Species: group of organisms that breed together and produce offspring • Share a gene pool • Speciation: formation of new species All different species!

  14. Speciation occurs with… • A separation of gene pools • Reproductive isolation: separation so that two organisms cannot breed • No offspring are produced

  15. 3 Types of Reproductive Isolation: • behavioral: different mating behaviors • Ex: meadowlark’s song • geographic*: physical barrier separating • Ex: Abert and Kaibob squirrels • temporal: reproduce at different times • Ex: orchid pollen release

  16. Testing Natural Selection • Peter and Rosemary Grant- Princeton • Tested Darwin’s hypothesis with finches • Needed to prove 2 things: • 1. natural genetic variations were present • 2. variations caused a difference in fitness • Success- more support for Darwin! Go Darwin!

  17. Descent with modification on the Galapagos • All species of finch came from 1 founding population: • 1. founders arrive from mainland • 2. geographic isolation (separates gene pools) • 3. changes to new gene pools- adapt to local environment • 4. reproductive isolation- 2 new species can no longer mate • 5. ecological competition- 2 species live together, become more different to increase survival

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